This invention relates to AC to DC conversion arrangements, by which is meant arrangements capable of converting an input electrical supply of alternating waveform to an output electrical supply in which an alternating component is at least substantially reduced (hereafter a "DC supply"). Preferably the output supply is electrically isolated from the input supply. Preferably also the input supply comprises the AC mains supply and the output supply comprises a substantially steady unidirectional voltage.
A well known and simple form of mains-powered AC to DC converter capable of converting a substantially sinusoidal input supply to a DC output, generally at a lower voltage level but with electrical isolation between the input and output, comprises a step-down transformer and a half-wave rectifier. Such converters, however, exhibit poor power factors and are thus lossy and inefficient, although some improvement in efficiency can be achieved by full wave rectification of the transformer's output. In any event, however, since the transformer must operate at the relatively low mains frequency, it is bulky and expensive.
Further increases in efficiency, coupled with a reduction in transformer dimensions, can be achieved by interposing a full-wave rectifier and a high frequency inverter between the mains supply and the transformer, and it is also known that an inductor and inverter duty-cycle modulator circuitry can be used to control the level of the output voltage or current.
Today's EMC standards place high demands on, inter alia, the input power factors of mains-powered equipment, with the result that the input current waveform needs to track closely that of the input voltage. It is known to provide, for this purpose, so-called boost or buck mode pre-converter stages between the mains input rectifier and the transformer driver inverter to draw a controlled current from the supply, or to use flyback converter or buck converter topologies that combine output isolation with input power factor correction.
The foregoing arrangements exhibit various undesirable features, such as high cost, size and/or complexity of components and it is an object of at least the preferred embodiments of this invention to provide an AC to DC converter arrangement which is economical, efficient and compact. Preferably the arrangement also exhibits a controlled output, a near-unity power factor and can accommodate a wide range of input voltages.